Calcium/Ca(AlCl.sub.4).sub.2 -oxyhalide/carbon cells have been studied because of their high energy density and greater safety as compared to lithium/oxyhalide cells. However, calcium cells have had no or little useability under load at low temperature of about -30.degree. C. which has impeded the development of a marketable battery. Although cathode performance has been acceptable at low temperature, the anode has not been able to handle moderate current densities and has thus been responsible for poor performance. As one has raised the temperature of discharge, the anode behavior or load voltage has been improved, allowing for more and more cell capacity as temperature approaches ambient.
When a current load is imposed on a cell, resistance causes warming of the cell. Therefore, if one can improve load voltage at either the anode or cathode for a few minutes, the cell will begin to heat. This would be especially true for a spirally wound battery, where the local buildup of heat in the center of the cell is significant. Heating will raise the cell temperature, improve cell voltage under load, and cell capacity will thus be extended.
The general object of this invention is to improve the low temperature performance of an alkaline earth-oxyhalide electrochemical cell for at least several minutes to allow cell heating, after which the cell will sustain acceptable load voltage and capacity on its own. A more particular object of this invention is to provide ultra-safe calcium thionyl chloride cells that will perform at about -30.degree. C. A still further object of the invention is to provide such batteries that can be used in place of the lithium-sulfur dioxide batteries in current use.
It has now been found that the aforementioned objects can be attained by including bromine in the electrolyte of a calcium thionyl chloride cell to permit discharge at about -30.degree. C. at 5 mA/cm.sup.2 for several minutes with a load potential above 2.0 volts. Without bromine, load potentials are below 2.0 volts within a minute after the load is imposed. The modest performance allows a fully developed spirally wound cell to build up heat internally, with the rise in temperature improving the load potential and allowing significantly more cell life or capacity.
The bromine provides the cell with sufficient initial operating potential and capacity so that the internally generated heat will subsequently allow continued cell performance even at -30.degree. C.